> From: Jeff Lasman
> One
> of our users has suggested we delay all inbound messages on port 25 by three
> seconds to help eliminate spam.> # Accept hosts who are polite enough to wait rather than just send, as spammers do

Exim already does that by default. Read description of the smtp_enforce_sync
main configuration option, then telnet to Exim, you'll see 3 seconds delay
between connection and SMTP banner. If a spambot or an abused proxy
tries to send HELO before the banner, Exim drops the connection.
Check how many messages in logs containing:

SMTP protocol synchronization error (input sent without waiting for greeting): rejected connection from

> # Accept hosts who are polite enough to wait rather than just send, as spammers do
> accept hosts = *
> delay = 3s